Quantum-type infrared image sensors and thermal infrared image sensors have been used for detecting infrared irradiation discharged from heat sources, such as from a human body, to obtain an infrared image of the heat source.
Previous quantum-type infrared image sensors typically include a thermal sensor made of a semiconductor material, such as HgCdTe. Such devices have both a narrow bandwidth and must be operated at a reduced temperature, e.g., about 77 K, typically by use of a liquid nitrogen source. Such a quantum-type infrared image sensor can obtain a minimum temperature resolution (Noise Equivalent Temperature Difference: NETD) of about 0.1 K. However, since the sensor device must be cooled by use of liquid nitrogen or the like, such sensor devices are large and expensive.
Thermal infrared sensors typically do not require such cooling. Thermal infrared image sensors typically include multiple pixels each comprising a thermosensing elements such as a bolometer. The thermal sensor detects changes in characteristic values (e.g., resistance values) of the thermosensing elements due to changes in the temperature, to produce an infrared image.
FIG. 5 shows a conventional thermal infrared image sensor comprising bolometers. The sensor shown in FIG. 5 includes nine pixels arranged in a matrix of three rows and three columns.
The pixels include vertical read switch elements (vertical switches) Qij (i=1-3, j=1-3) and thermosensing elements Rij (i=1-3, j=1-3), such as bolometers. Typically the bolometers have changeable resistances that vary with changes in temperature caused by infrared irradiation. The vertical switches, typically comprising MOS transistors, selectively output current signals received from the thermosensing elements Rij. When the vertical switches Qij are turned on, the current signals of the thermosensing elements Qij are read onto vertical signal lines.
A power supply voltage VRB is typically applied to one terminal of each thermosensing element Rij in each pixel. The other terminal of each thermosensing element Rij is connected to a drain of the corresponding vertical switch Qij. The source of each vertical switch Qij is connected to a vertical read line LVj (j=1-3) corresponding to each column of pixels.
Gates of the vertical switches Qij are typically connected to clock lines CL1, CL2, CL3 that are, in turn, connected to a vertical reading circuit VSR. When drive pulses are sequentially sent from the vertical reading circuit VSR to the clock lines CLi (i=1-3), the vertical switches Qij are sequentially turned on, row-by-row.
The vertical read lines LVj are connected to a horizontal read line LH through respective horizontal switches QHj (j=1-3). The horizontal read line LH is connected to an inverted input terminal of a current-voltage conversion amplifier AMP. A feedback resistor RF is connected between the inverted input terminal and the output of the amplifier AMP. A predetermined bias voltage VR is applied to a non-inverted input terminal of the amplifier AMP.
Gates of the horizontal switches QHj are connected to respective horizontal selection signal lines HSj (j=1-3) (horizontal lines). The horizontal lines HSj are connected to a horizontal scanning circuit HSR. The horizontal switches QHj are sequentially turned on by drive pulses sent from the horizontal scanning circuit HSR through the horizontal lines HSj, thereby performing a horizontal read operation.
To operate the thermal sensor shown in FIG. 5, the vertical reading circuit VSR turns on one row of the vertical switches (e.g., Q11, Q12, Q13) thereby selecting the corresponding thermosensing elements (R11, R12, and R13). The horizontal scanning circuit HSR sequentially turns on the horizontal switches QH1, QH2, and QH3. In this manner, the thermosensing elements R11, R12, R13 of the first row of pixels, are sequentially selected for horizontal scanning. Each of the thermosensing elements is sequentially selected to cause a current to flow.
More specifically, for example, a current flows from the power supply source VRB to the circuit of the horizontal read line LH via the thermosensing element R11, the vertical switch Q11, the vertical read line LV1, and the horizontal switch QH1. This current is then converted into a voltage signal by the current-voltage conversion circuit or amplifier AMP and the feedback resistor RF. The voltage signal is output from an output terminal VOUT. Since the thermosensing elements Rij have resistance values that change depending on temperature, a corresponding voltage signal output from the output terminal VOUT also varies with the temperature of each selected thermosensing element Rij.
As described above, signals are first read from the first row of pixels. In a sequential manner, signals of the second or next row are then read by the horizontal scanning circuit HSR. As currents flowing in the pixels are sequentially detected, an output signal representative of a thermal image corresponding to one screen is provided.
FIG. 6 illustrates another conventional thermal infrared image sensor of the prior art, wherein each thermosensing element of the sensor is configured as a bolometer. By way of example, the sensor shown in FIG. 6 includes nine pixels arranged in a matrix of three rows and three columns.
Each pixel 51 includes a bolometer RB serving as a thermosensing element. The bolometer RB has a resistance that changes as the temperature of the bolometer changes due to infrared irradiation. Additionally, each pixel includes a switch QP or a MOS transistor (p-type) operable to selectively output a current in the bolometer RB to each corresponding vertical read line 52a-52c. When the switch QP is turned on, the current signal is read from the bolometer RB.
A power supply voltage VRB is typically applied to one of the terminals of each bolometer RB. Drains of the switches QP are connected to the other terminal of each bolometer RB. Sources of the switches QP are typically connected to pixel columns via vertical read lines 52a-52c.
Gates of the switches QP are typically connected to each row of pixels and to clock lines 53a-53c. The clock lines 53a-53c are connected to a vertical reading circuit 54. Drive pulses .phi.V1-.phi.V3 are sent from the vertical reading circuit 54 to the switches QP and the switches QP are sequentially operated by pixel rows.
The vertical read lines 52a-52c are connected to a horizontal read line 56 through horizontal read switches (n-type MOS transistors) QH1-QH3. A current-voltage conversion circuit 58 (having an input intermittently grounded to a constant-voltage power supply VR) is connected to the horizontal read line 56. A current signal read from each pixel 51 onto the horizontal read line 56 is converted into a voltage signal by the current-voltage conversion circuit 58. The voltage signal is then sequentially output from an output terminal VO.
The gates of the horizontal read switches QH1-QH3 are connected to respective horizontal selection signal lines 55a-55c that are connected to a horizontal scanning circuit 57. The horizontal read operation of the pixels is controlled by drive pulses QH1-QH3 sent from the horizontal scanning circuit 57 to the horizontal read switches QH1-QH3. A parasitic capacitor CH is typically placed along the horizontal read line 56.
Operation of the thermal infrared image sensor shown in FIG. 6 is described with reference to a pulse timing chart shown in FIG. 7. Referring to FIG. 7, reading operations of the first row of pixels 51 (FIG. 6) are performed during periods t11 and t12. Reading operations for the second row of pixels 51 are performed during periods t21 and t22. Reading operations for the third row of pixels 51 are performed during periods t31 and t32.
The drive pulse .phi.V1 is set to a low level during period t11, and the switches QP of the first row of pixels are turned on. During period t12, high-level drive pulses .phi.H1-.phi.H3 are sequentially sent from the horizontal scanning circuit 57 to sequentially turn on the horizontal read switches QH1-QH3. Since the switches QP of the first row of pixels 51 have been turned on, current signals from the first row of pixels 51 are sequentially read onto the horizontal read line 56 via the vertical read lines 52a-52c. The current signals are converted into voltage signals by the current-voltage conversion circuit 58, and the voltage signals are sequentially output to the output terminal VO.
A current signal IRB, output from each pixel 51, may be expressed by Equation 1 below. Because the horizontal read line 56 is intermittently grounded to the constant-voltage power supply VR: EQU IRB=(VR-VRB)/RB (1)
wherein RB is the resistance value of the respective bolometer RB.
When the resistance value RB of the bolometer changes with increases in temperature, the current signal IRB corresponding to the increase in temperature is output from each pixel 51.
During periods t11 and t12, drive pulses .phi.V2 and .phi.V3 are at a high level, and the switches QP of the second and third rows of pixels 51 remain in an OFF state. For this reason, current signals from the second and third rows of pixels 51 are not output. At the end of period t12, the drive pulse .phi.V1 is set to a high level, the switches QP of the first row of pixels 51 are turned off, and the read operation for the first row of pixels 51 is completed.
The reading operation for the first row of pixels during periods t11 and t12 (described above) are similarly repeated for the second and third rows of pixels during periods t21 and t22 and periods t31 and t32, respectively. Following these operations, the reading operation with respect to all of the pixels is completed.
A pixel signal includes a background light component. In general, in prior-art thermal infrared image sensors, an NETD of about 0.1 K can be obtained. However, the background light component of the pixel adds heat of a level equal to about room temperature (i.e., about 300 K). In the prior art, the signal has been saturated by the background light component to amplify the signal; however, gain cannot be increased.
In order to avoid the above-described problem, the signal component and the background light component are separated by use of, for example, a differential amplifier placed outside the device. However, as is well known, the background light components of each of the pixels vary. Because prior-art methods of separating the signal component from the background light component occur outside the device, variations in the background light components of each of the pixels cannot be removed, and a fixed pattern of noise is generated.
In prior-art thermal infrared image sensors, a voltage is applied to both the ends of the bolometers serving as thermosensing elements to sequentially select the bolometers. Output current signals from the pixels are directly read on the horizontal read line. For this reason, when signals are read by the NTSC scheme (which serves as a standard television scheme having hundreds of thousands of pixels) a selective switching operation for pixels must be performed at several MHz. However, since time periods due to the resistances of the bolometers and floating capacitors CVj (j=1-3) of the read circuits (i.e., the vertical read line, the horizontal read line, and the like) are long, a high-speed read operation cannot be performed, and a read operation using the NTSC scheme has been difficult to obtain.
Previously, the resistance of each bolometer has been reduced to make it possible to realize high-speed reading. However, reducing the resistance has caused an increase in the current flowing in the bolometer, causing the elements themselves to generate heat. This element-generated heat serves as external disturbances to the ultimately produced thermal image. That is, such self-generated heat is equivalent to noise, and makes it impossible to detect a signal component at a high sensitivity level. Accordingly, a trade-off between the sensitivity and the reading speed has been established. That is, both the sensitivity and the reading speed cannot be optimized at the same time.
To solve the above difficulty, a plurality of reading circuits may be arranged, and the signals may be simultaneously read from the plurality of pixels, in parallel. However, the large number of signals must be processed with a peripheral circuit. The arrangement of the peripheral circuit is complicated and increases the cost of the sensor.